"Would you like Selwyn to adopt you?" asked my mother during my visit home to Cologne. It was summer 2003 and I was 33. Under normal circumstances I would have laughed, but this was serious.

Six years earlier my mother had been diagnosed with cervical cancer. She survived, but the cancer had returned and this time her prognosis was less optimistic. Her time was running out and she was concerned that the family might fall apart after her death. She feared that my stepfather, Selwyn, would be left lonely, especially because I had moved to Cardiff and was flying home only twice a year. And she worried that I would not have the same legal rights as my half-sister, Elizabeth, who is 22 years younger than me.

So instead of laughing, I tried not to cry as I replied: "Isn't it a bit too late for that?" After all, my stepfather had been in my life since I was nine, and this was the first time adoption had been mentioned to me. And if my parents had asked me this question when I was growing up, my answer would have been very different.

My mother, Veronika, was just 16 when I was born, and living in a small German town called Goch close to the Dutch border. When I was four she made the difficult decision to leave me in the care of my grandparents while she went to study in Cologne. A year later my grandfather died and my grandmother was left to raise me alongside my youngest aunt, who was just five years my senior. I was not a happy child. My mother only came home once a fortnight or during her holidays, and I missed her terribly. I also did not get on well with my grandmother, because I felt she favoured my aunt over me.

By the time I was nine the arguments between my grandmother and me were so bad that my mother took me to live in Cologne. She had met and moved in with Selwyn and my grandmother was pessimistic about the move; my mother was in the middle of her exams and she and Selwyn had to move to a bigger flat to accommodate me.

To begin with, it seemed as if my grandmother was right. The first year with my mother and newly acquired stepfather was very stressful - I was jealous and he was wary. After years of being separated from my mother I wasn't prepared for the fact I would have to share her. I also had to get used to a new school and a big city as well as this new father. The first couple of years were hard on Selwyn too. New to parenthood, my arrival changed his world dramatically. He believed I thought he was an unwelcome intrusion, and decided to stay in the background as far as my upbringing was concerned. He certainly never tried to impose himself as a father figure. However, my mother was naturally keen that my stepfather and I would get on. Selwyn started attending parent-teacher meetings with her and got involved in school trips. Gradually, our relationship improved.

During my teenage years Selwyn had a tremendous influence on me. He used to quiz me on the oldies playing on the radio and tell me the name of the song or artist. He is a lecturer in English literature and often discussed novels with me, influencing my choice of books. He inspired me to achieve good grades in English and I eventually studied the subject at the university where he lectured. When I broke up with my first boyfriend Selwyn was there for me. When I travelled through his home country of New Zealand nine years ago and my money was running out, he transferred extra cash into my account and was happy that I had a great time. It was while travelling that I met my now-husband, Paul.

In contrast, my biological father, who split from my mother before I was born, never cared about me and I only met him once, very briefly. I was keen to contact him after being inspired by one of my schoolfriends. She had tracked down her African-American biological father in spite of some family opposition and had never regretted it. So, when I was 20, I wrote to my "real" father. He didn't reply.

Despite this, I mustered all my courage and eventually rang him to arrange a meeting. It was a disaster. We went from a noisy pub to an even noisier disco where conversation was almost impossible. During our brief encounter he had nothing good to say about my family, and the final straw came when he said he had not replied to my letter because he wanted to know how serious I was about meeting him.

I never saw him again, and years later when I tried to track him down to get his consent for my adoption it proved impossible. His lack of interest made me realise that Selwyn was a much better father than he could have ever been.

Selwyn later told me that when I was a teenager my biological father had suggested he adopt me. He must have thought that as I had lived with Selwyn for so long it was a natural conclusion. But my mother was not happy with the proposition - she thought that my stepfather had already done enough. Now I was an adult, it was Selwyn who suggested it. After all that we had been through, I didn't have to think long about my answer. I said yes, but it didn't happen immediately. It was 2006 by the time the subject came up again. My mother was in a hospice and her wish to see the adoption settled had become urgent. During her last two weeks, the solicitor visited to arrange the necessary paperwork.

My mother was just 53 when she died. She left no instructions for her funeral so Selwyn and I were left to make the preparations and it brought us even closer. When we tried to imagine what she would have wanted we agreed on every little detail. As we organised things I noticed, for the first time, how the past months had taken their toll on Selwyn. He was very emotional when we were choosing a burial plot; he told me that he was reminded of how on their last holiday together, he and my mother had visited a famous cemetery in Paris, just a few months before her death when she was already weakened by the cancer. At the funeral we were too busy comforting everyone else, but at this moment we had time to share our own grief and we cried as we held each other.

Months later the adoption process had still not been completed and the bureaucracy seemed endless. Finally, the judge informed my stepfather that he couldn't decide whether German or British law applied and our case would have to be reviewed by a legal expert. The prospect of spending thousands of euros in legal costs for a report that might conclude the adoption was impossible was not a nice one. I asked Selwyn if this was really something he wanted to do. But he was determined.

In May 2007 I married Paul, and although it was Selwyn who gave me away I was not allowed to name him as my father on the certificate - so I left the space blank. That December the judge finally invited us both into his office. He wanted to know how serious we were about the adoption and asked questions about our relationship. He then told us that adult adoption doesn't exist in Britain and German law would apply. However, because I had married in the meantime, my husband had to sign a document that approved the adoption. Finally, in January 2008, my maiden name - which already had changed from Kaut to White - changed again to Jackson. I was adopted. My mother's wish had been fulfilled.

Has the adoption changed anything in my relationship with my stepfather? I think it has. Although we had already grown close over all these years, I feel more responsible and more protective of him now. As he gets older I worry about him more and visit as often as I can. I also feel that my sister, Selwyn and I are just a normal family like any other. When I speak to Elizabeth I call Selwyn "dad" - not stepdad. I just wish my mother was still here with us. She would have been happy to see that, despite living miles apart, our family has grown closer than ever.

• This article was amended on Tuesday 31 March 2009. The byline should have been Helen Kaut, not Helen Kaut-White. She was 33, not 37, in the summer of 2003. This has been corrected.